The present invention relates to a process for the preparation of di- and polyamines of the diphenylmethane series.
Di- and polyamines of the diphenylmethane series are understood as meaning compounds and compound mixtures of the following structure:
wherein n represents a natural number>2.
The continuous, discontinuous, or semi-continuous preparation of di- and polyamines of the diphenylmethane series, also called MDA in the following text, is described in numerous patents and publications. See e.g. H. J. Twitchett, Chem. Soc. Rev. 3(2), 209 (1974), M. V. Moore in: Kirk-Othmer Encycl. Chem. Technol., 3rd ed., New York, 2, 338–348 (1978). The preparation of these polyamines is conventionally carried out by reaction of aniline and formaldehyde in the presence of acidic catalysts. Aqueous HCl is conventionally employed as the acidic catalyst. According to the prior art, the acidic catalyst is neutralized by addition of a base, and thus, used up at the end of the process, and before the final working up steps (such as, for example, removal of excess aniline by distillation).
The di- and polyisocyanates of the diphenylmethane series, called MDI in the following text, are prepared by phosgenation of the corresponding di- and polyamines. The di- and polyisocyanates of the diphenylmethane series which are prepared in this way thereby contain the various isocyanate isomers and higher homologues thereof in the same composition as the polyamines from which they have been prepared.
In the course of the preparation of MDA, the acidic reaction mixture is then conventionally neutralized with a base. After the neutralization, the organic phase is conventionally separated from the aqueous phase in a separating container. The product-containing organic phase which remains after the aqueous phase has been separated off is then conventionally subjected to further working up steps, such as a washing with water, and then freed from excess aniline and other substances present in the mixture (e.g. further solvents) by, for example, distillation, extraction or crystallization.
Experience in the plant shows, however, the separating off of the aqueous phase from the product-containing organic phase after the neutralization and/or the subsequent washing, may be severely impaired by the formation of a third phase (rag or rag layer). This third phase is a stable, possibly voluminous intermediate phase, which occurs between the aqueous and the organic phase, and makes phase separation difficult. In the extreme case, this third phase even prevents phase separation completely. In the most adverse case for the process in the plant, the phase separation tank or tanks affected must be emptied completely and cleaned. The content of the phase separation tank or tanks then has to be worked up or disposed of with great effort, which is associated with considerable costs. Under certain circumstances this can also lead to the continuous production having to be interrupted.
The object of the present invention was, therefore, to provide a simple and economical process for the preparation of di- and polyamines of the diphenylmethane series, in which the separating off of the aqueous phase from the product-containing organic phase, after the neutralization and/or the subsequent washing, can be carried out simply and without trouble.